Air-to-air atmospheric exchanger for condensing cooling tower effluent

ABSTRACT

Heat exchanger packs having a first set of passageways for receiving a stream of ambient air and a second set of passageways for receiving a stream of warm water laden air is disclosed. The first set of passageways and second set of passageways being separate and permitting the warm water laden air stream to be cooled by the stream of ambient air so that water can condense out of the warm water laden air stream. Cooling tower configurations including the heat exchanger pack are disclosed for achieving effluent plume abatement, and capture of a portion of the effluent for replacement back into the cooling tower reservoir or as a source of purified water.

FIELD OF THE INVENTION

The present invention relates generally to water reclamation fromcooling tower effluent or other heat rejection devices. Moreparticularly, the present invention relates to method and apparatus forreclaiming water from cooling tower effluent to provide a source ofclean water, reduce water consumption of the cooling tower, and/or toreduce the cooling tower plume.

BACKGROUND OF THE INVENTION

In electricity generation using steam driven turbines, water is heatedby a burner to create steam which drives a turbine to createselectricity. In order to minimize the amount of clean water necessaryfor this process, the steam must be converted back into water, byremoving heat, so that the water can be reused in the process. In airconditioning systems for large buildings, air inside the building isforced passed coils containing a cooled refrigerant gas therebytransferring heat from inside the building into the refrigerant gas. Thewarmed refrigerant is then piped outside the building where the excessheat must be removed from the refrigerant so that the refrigerant gascan be re-cooled and the cooling process continued.

In both of the foregoing processes, and numerous other processes thatrequire the step of dissipating excess heat, cooling towers have beenemployed. In wet type cooling towers, water is pumped passed a condensercoil containing the heated steam, refrigerant, or other heated liquid orgas, thereby transferring heat into the water. The water is then pumpedto the top of the cooling tower and sprayed over a cooling tower mediacomprised of thin sheets of material or splash bars. As the water flowsdown the cooling tower media, ambient air is forced passed the heatedwater and heat is transmitted from the water to the air by both sensibleand evaporative heat transfer. The air is then forced out of the coolingtower and dissipated into the surrounding air.

Cooling towers are highly efficient and cost effective means ofdissipating this excess heat and thus are widely used for this purpose.A recognized drawback to cooling towers, however, is that under certainatmospheric conditions a plume can be created by moisture from theheated water source evaporating into the air stream being carried out ofthe top of the cooling tower. Where the cooling tower is very large, asin the case of power plants, the plume can cause low lying fog in thevicinity of the cooling tower. The plume can also cause icing on roadsin the vicinity of the cooling tower where colder temperatures cause themoisture in the plume to freeze.

Efforts have therefore been made to limit or eliminate the plume causedby cooling towers. Examples of such efforts can be found in thefollowing United States Patents:

U.S. Pat. No. 6,247,682 to Vouche describes a plume abated cooling towerin which ambient air, in addition to being brought in at the bottom ofthe tower and forced upwards through a fill pack as hot water is sprayeddown on the fill pack, is brought into the cooling tower throughisolated heat conductive passageways below the hot water spray heads.These passageways which are made from a heat conductive material such asaluminum, copper, etc., allow the ambient air to absorb some of the heatwithout moisture being evaporated into the air. At the top of the towerthe wet laden heated air and the dry heated air are mixed therebyreducing the plume.

U.S. Pat. No. 4,361,524 to Howlett describes a plume prevention systemin which the hot water is partially cooled before being provided intothe cooling tower. The partial cooling of the hot water is performedusing a separate heat exchanger operating with a separate cooling mediumsuch as air or water. As discussed in the patent, the separate heatexhanger reduces the efficiency of the cooling tower and thus shouldonly be employed when atmospheric conditions exist in which a plumewould be created by the cooling tower.

Another example of a system designed to reduce plume in a wet typecooling tower can be found in the “Technical Paper Number TP93-01” ofthe Cooling Tower Institute 1993 Annual Meeting, “Plume Abatement andWater Conservation with the Wet/Dry Cooling Tower,” Paul A. Lindahl, Jr.et al. In the system described in this paper, hot water is first pumpedthrough a dry air cooling section where air is forced across heatexchange fins connected to the flow. The water, which has been partiallycooled, is then sprayed over a fill pack positioned below the dry aircooling section and air is forced through the fill pack to further coolthe water. The wet air is then forced upwards within the tower and mixedwith the heated dry air from the dry cooling process and forced out thetop of the tower.

While the foregoing systems provide useful solutions to the wet coolingtower plume problem, they all require the construction of a complex andcostly wet and dry air heat transfer mechanism. A simple and inexpensivewet and dry air cooling mechanism is still needed wherein dry heated airand wet laden heated air can be mixed before passing out of the coolingtower to thereby reduce the plume.

Another recognized problem with cooling towers is that the water usedfor cooling can become concentrated with contaminates. As waterevaporates out of the cooling tower, additional water is added but itshould be readily recognized that contaminants in the water will becomemore concentrated because they are not removed with the evaporate. Ifchemicals are added to the cooling water to treat the water thesechemicals can become highly concentrated which may be undesirable ifreleased into the environment. If seawater or waste water is used toreplace the evaporated water, a common practice where fresh water is notavailable or costly, salts and solids in the water can also build up inthe cooling water circuit As these contaminants become more concentratedthey can become caked in between the thin evaporating sheets diminishingthe towers cooling efficiency.

To prevent the foregoing problem it is a regular practice to “blowdown”a portion of the water with the concentrated contaminants and replace itwith fresh water from the source. While this prevents the contaminantsin the cooling tower water from becoming too concentrated, there may beenvironmental consequences to discharging water during the blowdownprocess. Efforts have therefore been made to reduce the waterconsumption in cooling towers.

U.S. Pat. No. 4,076,771 to Houx, et al. describes the currentstate-of-the-art in reducing the water consumption in a cooling tower.In the system described in this patent both cooling tower evaporativeheat transfer media and a coil section that transfers heat sensibly areprovided in the same system. The sensible heat transfer of the coilsprovides cooling of the process water but does not consume any water.

While the foregoing patent represents a significant advancement overprior art cooling towers, it would be desirable if a mechanism weredeveloped for recapturing water from the plume for replacement back intothe cooling tower water reservoir which did not require a coil sectionfor sensible heat transfer.

A separate problem that has been noted is the desalination of sea water,and purification of other water supplies, to create potable drinkingwater. Numerous approaches have been developed to remove purified waterfrom a moist air stream. The major commercial processes includeMulti-Stage Flash Distillation, Multiple Effect Distillation, VaporCompression Distillation, and Reverse Osmosis. See “The DesaltingABC's”, prepared by O. K. Buros for the International DesalinationAssociation, modified and reproduced by Research Department Saline WaterConversion Corporation, 1990. Examples of systems that use lowtemperature water for desalination or waste heat include the following:

“Zero Discharge Desalination”, Lu et al, Proceedings from the ADA NorthAmerican Biennial Conference and Exposition, August 2000. This paperprovides information on a device that produces fresh water from a coldair stream and a warm moist air stream from a low grade waste heatsource. The fresh water is condensed along the walls separating the twoair streams. Also, a cold water is sprayed over the warm moist air toenhance condensation.

“Open Multiple Effect Desalination with Low Temperature Process Heat”,Baumgartner et al, International Symposium on Desalination and WaterRe-Use, Vol. 4, 1991. This paper provides information on a plastic tubeheat exchanger used for desalination that uses cold running water on theinside of the plastic tubes and warm moist air flowing over the exteriorof the tubes. The condensate forms on the outside of the cold tubes.

The foregoing show that there is a need for desalination systems forconverting sea water, or other water supply containing high levels ofcontaminants, into a purer water supply. A simple and cost effectivemeans of condensing the effluent of a cooling tower as a source of waterwould therefore be desirable.

SUMMARY OF THE INVENTION

In one aspect of the invention a heat exchanger is provided having afirst set of passageways formed for receiving a first stream of air. Asecond set of passageways for receiving a second stream of air is alsoprovided in the heat exchanger, the second stream of air being warmerthan said first stream of air. Each passageway of the first set ofpassageways is separate but adjacent to at least one passageway of thesecond set of passageways so that heat from said second air stream willbe absorbed by the first air stream. A reservoir for capturing moisturethat condenses out of said second air stream is also provided.

In another aspect of the invention a heat exchanger is provided havingtwo opposing walls configured with holes to allow for the passage of afirst air stream. Tubes are provided between a hole in the first walland a corresponding hole in the second wall for channeling the first airstream there through. Walls provided between at least two parallel edgesof one wall and the corresponding parallel edges of said second wallensure that a second air stream can be channeled passed said tubes tocondensed moisture out of the second air stream. In another aspect ofthe invention a method of reducing the moisture content of an air streamis provided wherein a first air stream having a flow rate between 10 and80 pounds of dry air per square foot per minute (pda/ft²/min) and arelative humidity at or above 90% is directed through a first set ofpassageways. A second air stream having a flow rate between 10 and 80pda/ft²/min and a dry bulb temperature at least five Fahrenheit degreesbelow the second stream is directed through a second set of passageways.Each passageway of the first set of passageways being separated from atleast one passageway of the second set of passageways by a thin heatconductive material. Heat from the second air stream is absorbed intothe first air stream and water condensed out of the second air stream iscaptured. In yet another embodiment of the invention, a cooling tower isprovided having a counterflow evaporative media and a water distributionsystem that distributes hot water over the counterflow evaporativemedia. A heat exchanger that absorbs heat from a first air stream into asecond air stream is also provided, the heat exchanger having a firstset of passageways and a second set of passageways. A fan in the coolingtower directs air through the counterflow evaporative media to createsaid first air stream and directs the first air stream, having a flowrate between 10 and 80 pounds of dry air per square foot per minute(pda/ft²/min) and a relative humidity at or above 90%, through the firstset of passageways. The fan also directs the second air stream having aflow rate between 10 and 80 pda/ft²/min and a dry bulb temperature atleast five Fahrenheit degrees below the second stream through the secondset of passageways. Each passageway of the first set of passagewaysbeing separated from at least one passageway of the second set ofpassageways by a thin heat conductive material. A reservoir is providedfor capturing water condensed out of the first air stream.

In another aspect of the invention a cooling tower is provided having afan at the top of the cooling tower for creating a negative pressureinside the cooling tower. A counterflow evaporative media is providedalong with spray heads that spray hot water onto the counterflowevaporative media. A heat exchanger having a first set of passagewaysfor passing an air stream from outside the cooling tower into the centerof the tower and a second set of passageways for passing an effluent airstream from the evaporative media is also provided in the heatexchanger. The air stream from outside the cooling tower absorbs heatfrom the effluent air stream and thereby condenses water out of theeffluent.

In yet another aspect of the invention, a cooling tower is provided witha fan at the top of the cooling tower for creating a negative pressureinside the cooling tower. A crossflow evaporative media and a hot waterdistribution system that sprays hot water onto the crossflow evaporativemedia are provided. A heat exchanger having a first set of passagewaysfor passing a first air stream from outside the cooling tower into thecenter of the tower and a second set of passageways for passing aneffluent air stream from said evaporative media is provided. The airstream from outside the cooling tower absorbs heat from the effluent airstream and thereby condenses water out of the effluent.

There has thus been outlined, rather broadly, the more importantfeatures of the invention in order that the detailed description thereofthat follows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the invention that will be described below andwhich will form the subject matter of the claims appended hereto.

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction and to thearrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable of otherembodiments and of being practiced and carried out in various ways.Also, it is to be understood that the phraseology and terminologyemployed herein, as well as the abstract, are for the purpose ofdescription and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conceptionupon which this disclosure is based may readily be utilized as a basisfor the designing of other structures, methods and systems for carryingout the several purposes of the present invention. It is important,therefore, that the claims be regarded as including such equivalentconstructions insofar as they do not depart from the spirit and scope ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a portion of a heat exchanger of apreferred embodiment of the invention.

FIG. 2 is a perspective view of a section of the heat exchanger of FIG.1 enlarged to show detail.

FIG. 3 is a graphical representation of a psychrometric chart for a heatexchanger.

FIG. 4 is a graphical representation of a psychrometric chart for aplume abatement process.

FIG. 5 is a graphical representation of a psychrometric chart for aplume abatement process with a moisture condensing heat exchanger.

FIG. 6 is a block diagram representation of a cooling tower inaccordance with a preferred embodiment of the invention.

FIGS. 7A and 7B are block diagram representations of a cooling tower inaccordance with another preferred embodiment of the invention.

FIGS. 8A and 8B are block diagram representations of a cooling tower inaccordance with another preferred embodiment of the invention.

FIG. 9 is a block diagram representation of a cooling tower inaccordance with another preferred embodiment of the invention.

FIG. 10 is a block diagram representation of a cooling tower inaccordance with another preferred embodiment of the invention.

FIG. 11 is a block diagram representation of a cooling tower inaccordance with another preferred embodiment of the invention.

FIG. 12 is a block diagram representation of a cooling tower inaccordance with another preferred embodiment of the invention.

FIG. 13 is a block diagram representation of a cooling tower inaccordance with another preferred embodiment of the invention.

FIG. 14 is a block diagram representation of a cooling tower inaccordance with another preferred embodiment of the invention.

FIG. 15 is an illustration of a tubular heat exchanger in accordancewith a preferred embodiment of the present invention.

FIG. 16 is a block diagram representative of a cooling tower inaccordance with another preferred embodiment of the invention.

FIG. 17 is a block diagram representative of a cooling tower inaccordance with another preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Heat Exchanger Pack

Referring now to the figures wherein like reference numerals indicatelike elements, in FIG. 1 there is shown a vapor condensing heatexchanger pack 10. The heat exchanger pack 10 is constructed of thinsheets 12 that are bonded together to form a pack that has a first path14 and a second path 16 for two different air streams. In a preferredembodiment, the two air streams enter the heat exchanger pack 10 atright angles to each other and are kept separate by the thin sheets 12.

The thin sheets 12 are a relatively thin synthetic resin material thatare shaped to assist in condensing vapor from a heated water laden airstream passing through passageways 14 and transferring heat to a coolair stream passing through passageways 16. In a preferred embodiment,the material is 0.005 to 0.040 inches in thickness but is preferably0.015 to 0.020 inches in thickness. The surface 18 may be textured toprovide extended surface area presented to each of the air streams witha minimal amount of resistance to the air stream flow. Examples oftexture patterns appropriate for such use can be found in U.S. Pat. No.5,944,094 to Kinney, Jr. et al. and U.S. Pat. No. 3,995,689 to Cates,the disclosures of which are incorporated herein by reference. Othertexture patterns may include but not be limited to textures such asdimples similar to golf ball texture and gridded texture similar to ascreen pattern embossed in the plastic sheet. This increased surfacearea enhances the heat transfer capabilities of the thin sheet andincreases the velocity fluctuations near the sheet surface, whichimproves the local mixing of the individual air stream. The increasedfluctuations and resulting local mixing of the air stream also improvesthe heat transfer capabilities of the sheet.

As depicted in FIG. 2, to prevent the two air streams from mixing duringthe heat exchange process, a seal 20 is formed in the thin syntheticresin sheet on a first edge. This seal is formed by the raised edges 22of the thin sheet material 12, on one edge of the sheets 12, that meetin the center of the air passageways 14, or in other words, is raisedone-half the width of the passageways 14. This edge seal 20 extendsalong the length of the air passageway 14 parallel to the airpassageways 16.

Similarly, a seal 24 is formed by the raised edges 26 of the thin sheetmaterial 12, on the edge perpendicular to the seal 20, that meet in thecenter of the air passageway 16, or in other words, is raised one halfthe width of the passageway 16. This edge seal 24 extends the length ofthe air passageway 16 parallel to the air passageway 14.

Although not shown, the edge parallel to seal 20 and the edge parallelto seal 24 are similarly bonded. Thus, perpendicular passageways 14, 16are formed within the heat exchanger pack.

One raised edge 26 extends positively off of the formed sheet and theother 22 downward or negatively. In this arrangement a single sheetcomponent can be used to form the basis of the entire heat exchangerpack. This is accomplished when assembling the pack by stacking thesheets 12 on top of each other and turning over every other sheet andlocating it on the previous sheet. Although only three passageways aredepicted, it should be readily recognized that in use the heat exchangerpack will have many perpendicular passageways and that any number ofpassageways can be formed using the sheets 12 as disclosed herein.

To maintain the opening of the air pathways, spacer knobs or buttons areformed in the thin sheet material. These buttons are spaced similarly tothe edge seal and extend either positively 28 from the formed sheet ornegatively 30 from the formed sheet a distance of half of the width ofthe air passage opening. In a preferred embodiment, the buttons 28 thatextend positively are conoidal in shape having a flat top surface whenviewed in the direction of the air flow through passageway 16. Whenplaced together the flat surfaces of the buttons of one sheet arearranged opposite the flat surface of the buttons on the adjacent sheet.Each of the positively protruding buttons 28 extends along a length ofthe thin sheet parallel to the direction of airflow. In a preferredembodiment, the buttons 30 which protrude negatively have the same shapeas the buttons 28 that protrude positively but are perpendicular inorientation. Again, the negatively protruding buttons 30 of one sheetare arranged opposite the negatively protruding buttons of the adjacentsheet. An alternate spacer embodiment which positions and interlocks thesheets may be found in Kinney '094.

The foregoing features are designed to maintain a consistent width ofthe air passageways and resist collapsing the passageways when adifferential pressure is applied between the two passageways. Theconfiguration of the buttons is also designed to provide a minimalresistance to airflow while providing sufficient structural resistanceto collapse of the passageways.

The width of each of the passageways for either the cool air stream orthe vapor laden air stream can be varied according to the designconditions of the particular task. Also, the cool air passageway 16 andthe vapor laden air passageway 14 do not necessarily have to be of equalwidths. Practically, for the particular tasks of the current invention,the passageway widths would be at a minimum of 0.5 inches wide and amaximum of 3.0 inches wide with a preferred width between 1.0 inches and1.5 inches.

The overall dimensions of the completed pack of thin sheets are alsodependent on the particular design task associated with the invention.However, the minimal pack size envisioned for the design is 2 feet by 2feet and the maximum is 6 feet by 24 feet.

The air entering the face of a heat exchanger pack is characterized bythe mass flow over the gross face area. Typically this is expressed aspounds of dry air per square foot of area per minute (pda/ft²/min). Inthe preferred embodiment, each set of air passages has a mass flow ratebetween about 10 pda/ft²/min to about 60 pda/ft²/min.

The temperatures of the warm moist air stream for the preferredembodiment of the three processes, water conservation, waterpurification, and plume abatement, are typical of those experienced bycooling towers and other waste heat rejection devices. Thesetemperatures would range from a maximum of about 150 degrees F. to aminimum of about 40 degrees F. Evaporative cooling towers typicallydischarge air that is saturated or nearly saturated (about 100% relativehumidity). Similar evaporative devices that supply air with a relativehumidity of about 90% or higher are feasible for this invention. Airstreams with relative humidities below about 90% require significantsensible heat transfer to cool the air streams to their respective dewpoints. Condensation can only take place after the air stream reachesthe saturation curve at the dew point.

For the preferred embodiment the operating pressures for the heatexchanger pack will be about the same as typical cooling tower operatingpressures, in a range of +/−6 millibars. In general cooling towersoperate at or near atmospheric pressure. Cooling towers have axial fansand/or blowers, also known as centrifugal fans, that create slightchanges from atmospheric pressure to generate flow through the packingmedia, spray, and drift eliminators. These different components cause arestriction to the air flow by friction and velocity differentials hencea pressure change from atmospheric is required to force the air throughthe tower. These pressures are typically in a range of +/−3 millibarsfor axial fan systems and +/−6 millibars for systems with a blower. Itis customary to consider such cooling tower systems operating at theserelative small pressure differentials to be operating at atmosphericpressure.

General Condensation Process

As described, the vapor-condensing heat exchanger is arranged in a packwith passageways for two different air streams. In the passageways 16cool air is delivered from an outside source or from the surroundingambient air mass. The method of obtaining the cool air is dependent onthe specific application for the device. The cool air temperature willtypically be significantly below the air mass temperature of the airstream in the opposing passageways 14. In the opposing passageway 14warm moist air is delivered into the path. The warm moist air istypically saturated with water vapor or has a dry bulb temperature thatis at or near the resulting wet bulb temperature. This air mass issimilar to that generated by a cooling tower which is used to rejectwaste heat from a process. However, other processes and methods thatgenerate a similar warm moist air stream can be used for the input intothis device such as an evaporative condenser.

As shown in the psychrometric chart of FIG. 3, the warm moist air is ata point 32 on the depicted saturation curve. The location of the point32 on the saturation curve where the warm moist air is indicates that itis 100% saturated with water vapor at a high temperature. The cool airentering the other passageway is located at a point below the saturationcurve 34. The location of the cool air on the psychrometric chartindicates that it is at a lower temperature than the entering warm air.The moisture content associated with this air stream is generally notrelevant to the functionality of the device. In the case of plumeabatement, however, the moisture content of the entering air effects notonly the moisture content of the “mix”, but also the tangency of the mixline.

As the two air streams pass through the heat exchanger the warm airstream is cooled and the cold air stream increases in temperature.Because the two air streams do not physically contact each other, thecool air stream is heated in a way that no moisture is added or removedfrom the air stream. This is known as a sensible heating of the airstream. As noted on the psychrometric chart, upon exiting the heatexchanger the cool air has an increased temperature but the moisturecontent has been maintained constant 36.

The warm moist air is cooled from its initial point on the saturationcurve 32 to a lower temperature. As the warm moist air mass is cooledthe moisture content of the air stream must be reduced. Since the airstream is 100% saturated, water will condense out of the air stream andthe resulting decrease in temperature will follow the 100% saturationcurve to the new cooler temperature 38. The amount of heat lost in thewarm saturated air stream must equal the amount of heat gained in thecool dry air stream.

Desalination research led to the serendipitous discovery that theexiting dry air of the air to air exchanger was much higher thanexpected. This discovery makes possible plume abatement with a devicepreviously assumed inadequate. Conventional wisdom suggested that anair-to-air heat exchanger for plume abatement is much less effectivethan a water-to-air heat exchanger such as coils or the plastic heatexchanger as disclosed by Kinney in '094. Cool ambient air is drawn fromoutside of the tower and heated sensibly. The heat source for warmingthis air would seem to favor water over air because of it's much largermass. For example the plastic heat exchanger in '094, typically has aflow rate of 20 gpm/sf or more. The mass flow rate then is typically 20gpm/sf×8.33 lbm/gallon=167 lbm/sf/min or more. The air-to-air heatexchanger as discussed above operates in a range from 10 to 80pda/sf/min. The total mass flow is determined by multiplying the dry airrate times (1+w_(s)) in which w_(s) is the humidity ratio. Assuming 100°F. saturated air, the humidity ratio, w_(s) is 0.0432. The mass of thisair stream varies from 10.4 to 83.5 lbm/sf/min. Therefore, the waterflow mass of the '094 plastic heat exchanger is typically several timesgreater than the air flow mass of the present invention. For comparableamounts of dry heat the air to air exchanger would seem to require achange in temperature of both air streams of several times that of thewater stream in the '094 heat exchanger. This was not thought to bepossible unless the surface was increased several times the surface areaof the water-to air heat exchanger to accomplish the same heat transfer.Therefore, the size of the air-to-air heat exchanger would seem toincrease to unmanageable or uneconomic proportions. However, using theheat exchanger of 10, FIG. 1., described previously the warm moist airstream is subject to a condensation process. In the condensationprocess, warm air comes in contact with a cool surface and watercondenses out of the air. In this process both sensible and latent heatare released and the absolute humidity is reduced. Since both latent andsensible heat is transferred in the device it becomes much moreefficient than previously thought possible.

In the passageway with the warm moist air 14, when the vapor comes incontact with the cool surface of the cool passageway, droplets ofcondensate are formed on the surface of the passageway with the warmmoist air stream. These droplets are a result of the warm moist airbeing cooled and the resulting moisture reduction of the air stream.These droplets coalesce on the sheet and flow down the warm moist airstream passageway surface of the sheet. The moisture that condenses ontothe sheet can either be collected at the base of the sheet or returnedinto the original source. The use of this water will be discussedfurther below.

Processes for Heat Exchanger

A. Water Conservation for Cooling Towers

As discussed in the previous section, warm moist air flowing through theheat exchanger passageway is cooled and the moisture content is reduced.The reduction in moisture content of the warm air causes droplets to beformed on the warm air passageway of the sheet. These droplets coalesceand fall from the bottom of the sheet. The water reclaimed from themoist air stream can be used to reduce the water consumption of acooling tower apparatus.

Cooling towers reduce the temperature of process water through anevaporative process and thus provide a place to remove heat from asystem. The heat removed is typically not useful for other processes andlabeled “low grade waste heat” and is released to the surroundingatmosphere. Through the cooling tower process a certain percentage ofthe process water that is circulating through the system is lost due toevaporation. The amount of water lost through the evaporation process istypically between 0.5% to 3% of the total flow rate. Generally this isroughly 0.8% for every 10° F. of cooling of the process water. This lossof water can be costly to operators of cooling tower apparatuses.

The water leaving the tower through evaporation is in a pure vaporstate, therefore, other contaminants such as solids, dissolved solids,salts, etc., are left in the process water. Over time, as pure water isremoved these contaminates build up in the process water. To reduce thecontaminants a certain percentage of the process water is removedcontinually. The water removed from the system is called blowdown.Therefore, to operate a cooling tower water must be added to bothcompensate for the evaporation of the water and the required blowdown.In many instances this water is difficult to discharge directly into theenvironment because of the quality of the chemical laden water and theincreased regulations associated with discharging water. Therefore,there is a significant economic advantage in reducing the amount ofblowdown.

With the air to air heat exchanger 10, FIG. 1, described above waterreclaimed from the warm moist air stream can be put back into thesystem. This will in effect reduce both the evaporation of the tower andthe required blowdown of the system. Configurations of a cooling towerincorporating this heat exchanger will be described below. Since thewater returned to the cooling tower system is nearly pure water, in manyinstances, it may be of better quality than the original make-up water.This improved water quality could also potentially reduce the amount ofchemicals required for the cooling tower process.

In order for the heat exchanger to operate effectively and return waterback into the system, the air temperatures entering into the cool airpassageway must be below the warm air entering the opposing passageway.For a water conservation apparatus, as the two temperatures becomecloser to the same value the amount of water returned to the basin willbe less. If the cool side of the heat exchanger is supplied with ambientair temperatures and not cooled by other means, the heat exchanger willreturn more water when the temperatures are cooler or during winteroperation. During summer operation the heat exchanger will return lesswater. Typical values of water returned back into the basin will rangefrom 40% of the evaporated water during the winter months to 3% of theevaporated water during summer operation. Water returned on an annualbasis would be around 10% to 30% depending on the location. Table 1below shows the percentage of evaporation water reclaimed for variouslocations in summer and winter. The numbers provided are for maximumwater reclaimed from cooling tower effluent based on local conditionsand a power plant duty of a 25-degree Fahrenheit range.

Wyoming Nevada Florida New York Saudi Arabia Summer 15% 3% 11% 16% 3%Winter 40% 23% 21% 32% 14%

B. Water Purification and Desalination

A cooling tower generates warm moist air during the evaporation heatrejection process. This warm moist air contains nearly pure vapor and isfree of most contaminants such as solids, dissolved solids, salts, andchemicals. A significant portion of this pure vapor can be recoveredwhen this type of heat exchanger is employed. In addition to recyclingthe water back into the cooling tower reservoir, the pure vapor whenconverted back into the water state can be used for other applicationsthat require a source of clean water. Because of the expense associatedwith providing process water for cooling towers, often the make-up waterused is either salt water from ocean sources or wastewater from anindustrial process. When employed as a water reclamation device thisheat exchanger is capable of converting water that is otherwiseundesirable because of the quality of water.

While not pure, the resultant water will be free of most impurities.Viruses, biological impurities, and a small amount of dissolved solidsmay be entrained in the vapor. Also, a small amount of process coolingwater may also be entrained into the moist air stream and contaminatethe condensed water. This type of carry over is termed “drift” in thecooling tower industry. A secondary purification process may be employedto obtain further levels of desired water quality. The advantageprovided by the present process can be seen in the case of sea waterdesalination to create potable water. In the case of desalination of seawater, one of the most expensive steps in the process is removing thesalts. The foregoing cooling tower reclamation process could be used toreduce the salt content considerably so that a less expensive processcould be used for the final purification of the water. An example of aprocess that can be used for the final purification process is reverseosmosis.

The process of recovering the water for other uses is essentially thesame as has been described previously in the water conservation sectionabove with the exception that the water recovered from the heatexchanger pack can be collected in a separate basin. Details of acooling tower application with a recovery basin are described below.

As with the water conservation tower, if the surrounding air is used asthe source for the cool side temperatures, as air temperatures increaseduring the summer months the production of clean water will decrease.Typical water recovered from this system will be 20% to 25% of the totalwater evaporated on an annual basis. If a source of either cold air orwater is available more water could be reclaimed from the system. Forexample, if a source of cold ocean water is available it could be usedto cool the incoming air in the cold passageway of the heat exchanger.As the temperature difference increases between the warm and cold sideof the heat exchanger sheet the condensation will increase and thus moreclean water will be generated. A configuration that would improve therate of clean water production when a source of cold seawater isavailable will be described below.

The water purification device is well suited for use in a cooling towerbecause of the generation of warm moist air, however other devices thatgenerate warm moist air could also be used in conjunction with thisdevice.

C. Plume Abatement for Cooling Towers

The heat exchanger of the present invention can also be used to reducethe visible plume of a cooling tower. This process is essentially thesame process as the water conservation process. The only difference isthe cold air heated in the cold side passageway is mixed with the warmmoisture laden air stream. The mixture of these two air streams caneffectively reduce the presence of the visible plume by an approachdifferent than typical plume abatement towers.

A typical method used to reduce the visible plume in a cooling tower isdepicted on the psychrometric chart of FIG. 4. As depicted in the chart,effluent air from the evaporative section of a cooling tower is warm100% saturated air 40. Warm water from the heat source is also sentthrough a coil or other heat exchanger located on the side of the tower.The warm water is used to heat the ambient air 42. Air is then pulledthrough both the evaporative heat section and the water/air heatexchanger. The ambient air 42 that flows through the water/air heatexchanger is heated without any change in the moisture content (i.e.sensible heat transfer) 44. The warm dry air 44 then exits from theair/water heat exchanger.

The warm dry air stream 44 exiting the air/water heat exchanger is thenmixed with the moist air stream 40 exiting the evaporative section ofthe cooling tower. The mixture of these two air streams results in anair stream 46 which has the property that when the exiting cooling towerair stream 46 temperature and the ambient air temperature 42 areconnected with a line on a psychrometric chart, the connecting line 48does not cross over the 100% saturation curve. If the connecting line 48were to cross over the 100% saturation curve when the ambient andexiting air are mixed, condensation of the water vapor from the airstream of the evaporative section would occur creating a visible plumeor fog. The area above the 100% saturation curve is the super saturatedarea and is also termed the fog area. Therefore, systems are designedsuch that when the properties of the air mass exiting the cooling towersand the ambient air mass properties are mixed no visible plume willoccur for a given design condition.

Using the air to air heat exchanger 10, FIG. 1, of the presentinvention, the typical process is modified by reducing the moisturecontent of the air stream from the evaporative section and providing asource of warm dry heat to reduce the plume. The reduction in moistureof the warm moist air stream is a reduction in the absolute humidity ofthe air stream. The water content of the air from the evaporativesection of the cooling tower is reduced by use of the air to air heatexchanger as described above. The source of the warm dry air is theambient air that is heated in the heat exchanger from the cold airpassage.

The plume abatement process with the air to air heat exchanger of thepresent invention is depicted in the psychrometric chart of FIG. 5. Asthe exiting air from the cooling tower evaporative section 40 passesthrough the heat exchanger the temperature and moisture content arereduced 50. The ambient air, 42, is heated in the opposing passagewayresulting in a warmer dry air stream 52. The two air streams are mixedtogether forming a resultant air mass 54 below the saturation curve.When the ambient air mass 42 is mixed with the air mass from the mix ofthe two air streams 54 in the cooling tower the properties do not crossover into the super saturation area of the curve or the fog area. Thisis depicted by a line 56 connecting the ambient air mass 42 and themixed air mass 54 on the psychrometric chart.

The foregoing method for plume abatement is very effective for thereduction of the plume because moisture that could cause a plume to formis partially removed from the tower before entering the surroundingambient conditions. The method is also less complicated because there isno water used in the heat exchanger system. Since no water is used inthe heat exchanger it eliminates the complexity of providing anotherpiping system for the cooling tower.

Cooling Tower Configurations

A first preferred embodiment of a cooling tower 58 employing the heatexchanger described above is depicted in FIG. 6. In this configurationthe heat exchanger 10 is located above the evaporative media 60 in acounterflow arrangement. This placement of the heat exchanger would bebest suited for the water conservation and plume abatementconfigurations. The process employed by this cooling tower is asdescribed below.

Hot water from the heat source is pumped through a conduit having sprayheads 62 and sprayed over the evaporative media 60. An axial fan (orfans) 64 assist airflow of cool ambient air 66 through the evaporativemedia. In the evaporative media 60, the air is heated and moisture isevaporated into the air stream. The heated water laden air is thendirected through air flow passageways 14 of the heat exchanger 10.Ambient air 68 is also directed through separate passageways 16 of theheat exchanger perpendicular to the flow of the heated water laden air.The cool ambient air 68 generates a cool surface on the heat exchanger10 for the vapor to condense on. The condensate 15 will fall from theheat exchanger back into the main water collection area of the coolingtower. Condensate droplets size is exaggerated in the Figures forclarity. The two air streams 70, 72 exiting the heat exchanger 10 arecombined near the fan inlet.

The air-to-air heat exchanger 10 when incorporated into a cooling towerwill create a resistance to the fan 64. The increased resistance willrequire that the power be increased to the fan 64 in order to maintainthe same flow rate through the cooling section with the addition of theheat exchanger 10. As depicted in FIGS. 7A and 7B, during operationaltimes when more cooling tower performance is necessary, air vent doors74 located in the tower may be opened. When opening these doors 74 asignificant amount of air will bypass the heat exchanger 10 and godirectly to the fan 64. This will reduce the air resistance created bythe heat exchanger 10 and increase the amount of air that will flowthrough the cooling tower media 60. By increasing the airflow throughthe media 60, the performance of the cooling tower will increase.However, when bypassing the heat exchanger 10 the water conservation,water purification, and plume abatement processes are halted.

An alternate embodiment of the doors are depicted in FIGS. 8A and 8B. Inthis configuration the doors 76 not only provide a method to allow thewarm moist air to bypass the heat exchanger 10, but also provides a wayto close off the cold side of the heat exchanger. In effect, becomingdamper doors.

Another method to reduce the amount of resistance in the heat exchanger10 is to increase the flow area of the heat exchanger pack. As depictedin FIG. 9, in order for two different air streams (warm moist air andcold ambient air) to flow through the single fan 64 of a cooling tower,a portion of the flow area from the cooling tower media is blocked off.Since a portion of the flow area is blocked off the velocity of the airstream must be increased accordingly. This increased velocity of theflow creates more resistance when passing through the heat exchanger 10.In order to reduce the resistance, the heat exchanger flow area may beexpanded by the amount of the blockage. In this configuration the heatexchanger pack 10 is in effect cantilevered beyond the cooling towermedia 60. This reduces the velocity of the warm moist air through theheat exchanger and reduces the amount of pressure drop in the system.

A third way to configure the heat exchanger 10, as depicted in FIG. 10,is to tilt the heat exchanger pack 10 upward 80 toward the fan 64. Thisconfiguration would provide an increased flow area for the heatexchanger and reduce the pressure drop as described previously. Thisconfiguration would also provide an improved air path for the airflowing on the inward-facing portion 68 of the heat exchanger 10 (coldpath), since the outlet of the pathway is positioned more towards thefan 64. The improved air path will result in less resistance andpressure drop for the heat exchanger cold side. Tilting heat exchanger10 may also be accomplished without cantilevering heat exchanger 10beyond cooling tower media 60.

In the configuration of FIG. 11 the length of the heat exchanger pack 10has been reduced in the upper sections 82. In this configuration thepressure drop of the system will be reduced because there is less heatexchanger media for the warm moist air to travel through. It will alsoprovide better mixing of the moist air stream and the dry air stream.The mixing of the two air streams is important in the plume abatementprocess in order to ensure that warm moist air does not mix with thecold ambient air to form a fog. Similarly, as depicted in FIG. 12, thelower portions of the heat exchanger pack 10 can be reduced 84 to reducepressure drop.

In an alternate embodiment of the cooling tower, the counterflowevaporative media is replaced with a crossflow media 86 as depicted inFIG. 13. The heat exchanger media 10 is located in the path of theexiting wet air stream in the plenum of the crossflow cooling tower. Theplacement of the heat exchanger 10 and evaporative media 86 in thisconfiguration would be best for the water purification and plumeabatement processes. The operation of this cooling tower is as describedbelow.

Hot water from the heat source is pumped to water distribution system 88and distributed over the crossflow evaporative media 86. An axial fan 64assists airflow of the ambient air 90 through the evaporative media 86and through the inward-facing panel 16 of the heat exchanger 10. Aircurrents exiting the evaporative media 86 are directed upward throughthe outward-facing panel 14 of the vapor-condensing media (heatexchanger) 10. The cool ambient air 90 condenses the vapor on theoutward-facing panel. The condensate falls from the heat exchanger backinto reservoir 92 where it can be collected for other uses or returnedback into the main circulating water system. The air streams from boththe inward-facing panel and the outward-facing panel 94, 96 are combinednear the fan inlet.

It is to be further understood that the doors 74 and 76 as shown inFIGS. 7A, 7B, 8A, and 8B for counterflow cooling towers may be readilyincorporated in crossflow cooling tower configurations. Furthermore, thetilting of heat exchanger pack 10 and the stepping of the heat exchangerpack 10 as shown in FIGS. 10, 11, and 12 for counterflow cooling towersmay be readily incorporated in crossflow cooling towers.

During operation of the system as a water purification or desalinationsystem the ambient temperatures may not be cold enough to provide thedesired output of clean water from the condensation process. In order toboost the output of clean water from the heat exchanger 10 a secondarysystem may be required to reduce the temperature entering into the coldside of the heat exchanger 10. As shown in FIG. 14, another bank ofcooling tower heat transfer media 98 may be placed in front of the coldside entrance of the heat exchanger 10. The cooling tower media 98 wouldbe sprayed with cold water to chill the incoming air. A possible sourcefor the cold water may be an ocean source or other large body of waterthat is cooler than the ambient dry bulb. If the wet bulb temperature islow, the cold water source does not necessarily have to be significantlycolder than the ambient dry bulb. The air would then enter in thecooling tower media and the temperature of the air reduced beforeentering into the cold side of the heat exchanger.

In an alternate embodiment depicted in FIG. 15, a tubular heat exchanger100 is used to replace the thin resin synthetic sheet pack 10. Thistubular heat exchanger will provide the same type of thermodynamicproperties as the thin resin synthetic sheet pack. The tubes 102 of thetubular heat exchanger could be made from a thin synthetic material asthe previously described heat exchanger or possibly a corrosionresistant metal such as galvanized stovepipe. These tubes 102 would beattached to a sheet 104 with holes so that the cold ambient air flowinginside the pipes 106 was separated from the warm moist air flowing overthe pipes 108. In a preferred embodiment, the tubes 102 are six inchesin diameter. The cooling tower configurations used with this type ofheat exchanger 100 are the same as shown previously.

In alternate embodiments of the cooling tower for counterflow, FIG. 16,and crossflow systems, FIG. 17, outside ambient air may be ducted toheat exchanger packs 10 located in the plenum area through one or moreducts 10. The packs would typically be in a staggered diagonal pattern.In this pattern the packs are not stacked directly above each other,thereby reducing the total pressure drop in the system. This embodimentreduces the total amount of heat exchanger 10 required by supplying coldambient air to each heat exchanger section thus creating maximum heattransfer in each heat exchanger section. In this configuration, thegeometry provides better mixing by intermingling the two airstreams.This will assist in plume reduction.

Gas to gas heat exchangers that transfer heat between two different gasstreams are commonly used in industrial and power generation processes.One type of gas-to-gas heat exchanger is called a plate-fin heatexchanger. These heat exchangers are usually made of metal and consistof a flat sheet separated by a series of corrugated sheets. Thecorrugated sheet serves to provide structural support to the heatexchanger and provide increased heat transfer by changing the flowstructure in the boundary layer and increased heat conductivity to theseparating plate (fin). The separating sheet, also known as the partingsheet, separates the two air streams and transfers the heat between thetwo gas streams by heat conductivity. See “Process Heat Transfer”,Hewitt, Shires, and Bott, CRC Press, Inc. 1994.

An advantage of the heat exchanger of the present invention is itslighter weight. For the preferred embodiment shown in FIG. 16, theoperating weight for a tower with 6′ bays is about 1100 lbs. Theoperating weight of an equivalently performing plastic heat exchanger,such as that of the Kinney '094 patent, is about 2200 lbs. Furthermore,the invention of '094 concentrates the weight at the outboard columns,whereas the weight of the heat exchanger in FIG. 16 is spread over 3bays. This reduces the amount of load added to individual columns. Lessweight or mass is also desirable for seismic design.

The present invention provides economic advantage over conventionalplume abatement and water conservation. As previously mentioned theair-to-air heat exchanger avoids the cost of having to pipe hot water tothe dry section of the cooling tower. Not only is the cost of the pipingavoided, but also the additional cost of pumping the water over the drysection is avoided. However, the fans experience an increase in staticpressure due to pulling the wet air stream through the air-to-air heatexchanger. The present invention requires approximately the same amountof power when compared to conventional 2 pass coils with a siphon loopto minimize head or less power when compared to single pass coils or theinvention by Kinney in '094. In the later case the total power savingcan amount to about 15′ of head which for 200,000 gpm tower flow isabout 900 horsepower. At $0.03/kw-hr this is a savings of about $175,000year.

Of more importance than the power savings are the maintenance andrequired water quality cost savings. Coils typically have 1″ to 1.25″diameter tubes. Larger tubes are typically not sufficient for therequired heat transfer. Water quality must be sufficient to preventfouling and plugging of the tubes. In the case of seawater or salt waterthe conventional finned tubes must be made of premium materials. Thismay be avoided by using the plastic heat exchanger as disclosed byKinney in '094, the disclosure of which is incorporated herein byreference. However, the heat exchanger water passages in Kinney '094 aremore restrictive than coils. If the water quality is not sufficient,filtration and or chemical treatment must be employed to improve andmaintain water quality. This can be expensive. The present inventionavoids the cost of improving and maintaining water quality. The moisturein the wet air stream is nearly pure which will not foul the air to airheat exchanger. Water quality less than has been thought possible forplume abatement or water conservation may be used with the presentinvention

Also, some cooling tower applications may have water with debris largerthan the heat exchanger passageways which would plug the passageways. Anexample is a “once through” power plant application in which water isextracted from a river or other body of water, heated by passing throughthe condenser, and then sent to a cooling tower before discharging backinto the body of water. The wet section of the cooling tower may havesplash fill and large orifice water distribution nozzles such asdisclosed in U.S. Pat. No. 4,700,893 issued to the present assignee. The'893 invention has been commercialized with 1.875″ and 2.5″ diameterorifices and could theoretically be larger. Therefore, water with debrislarger than previously thought possible for plume abatement can be used.

The wet section of the cooling tower may have splash fill and largeorifice water distribution nozzles such as disclosed by Bugler in U.S.Pat. No. 4,700,893, the disclosure of which is incorporated herein byreference. Thus fouling maintenance and water quality improvement costsare avoided. This can have an economic impact of $1,000,000 per year ormore on a large power plant tower.

Finally, the initial capital cost of the present invention is less thanthat of the prior art. Plume abatement towers typically cost 2 to 3times the cost of a conventional wet only tower. For a large power plantinstallation the plume abatement tower may cost $6,000,000 or more. Thesavings of the present invention can be $1,000,000 or more overconventional coil technology.

For desalination the cost per 1000 gallons of water is about $1.50compared to $4 with multi-stage flash desalination and $3 for reverseosmosis. The present invention requires secondary treatment to producepotable water. This adds about $0.50/1000 gallons. For a plant producing5 million gallons per day, this process can save $5,000 to $7,500 perday or about $2,000,000 annually.

The present invention provides plume abatement as a by-product on towersdesigned for desalination at no cost. Alternately, for cooling towerapplications requiring plume abatement, desalination can be a by-productfor the very little cost of collection by employing this invention. Themany features and advantages of the invention are apparent from thedetailed specification, and thus, it is intended by the appended claimsto cover all such features and advantages of the invention which fallwithin the true spirits and scope of the invention. Further, sincenumerous modifications and variations will readily occur to thoseskilled in the art, it is not desired to limit the invention to theexact construction and operation illustrated and described, andaccordingly, all suitable modifications and equivalents may be resortedto, falling within the scope of the invention.

What is claimed is:
 1. A method of reducing the moisture content of anair stream traveling through a cooling tower having evaporative media,comprising the steps of: directing a first air stream having a flow ratebetween 10 and 80 pounds of dry air per square foot per minute(pda/ft²/min) and a relative humidity at or above 90% through a firstset of passageways, directing a second air stream having a flow ratebetween 10 and 80 pda/ft²/min and an entrance dry bulb temperature atleast five Fahrenheit degrees below the first stream through a secondset of passageways each passageway of said first set of passagewaysbeing separated from at least one passageway of said second set ofpassageways by a thin heat conductive material; absorbing heat from saidfirst air stream into said second air stream with a heat exchanger;condensing water out of the first air stream; capturing the watercondensed out of said first air stream in a reservoir; and preventingeffluent from passing around the heat exchanger by using a coveringdisposed at the bottom of the heat exchanger.
 2. The method of claim 1wherein said step of directing a first air stream and said step ofdirecting a second air stream are accomplished using a single airdirecting device.
 3. The method of claim 2 wherein the air directingdevice is a fan.
 4. The method of claim 3 wherein the differentialpressure inside each first passageway and each second passageway isbetween positive six millibars and negative six millibars.
 5. The methodof claim 4 wherein the differential pressure inside each firstpassageway and each second passageway is between positive 3 millibarsand negative 3 millibars.
 6. The method of claim 4, wherein said firstair stream is obtained from a cooling tower effluent.
 7. The method ofclaim 6, wherein said second air stream is obtained from the ambient airoutside the cooling tower.
 8. The method of claim 6, further comprisingthe step of returning the captured water to the cooling tower reservoir.9. The method of claim 6, further comprising the step of purifying thecaptured water.
 10. The method of claim 9, wherein the step of purifyingthe captured water is performed using a reverse osmosis process.
 11. Themethod of claim 1 wherein the first set of passageways are formedthrough cylindrical tubes and wherein the second set of passageways arethe spaces around the cylindrical tubes.
 12. Apparatus for reducing themoisture content of an air stream traveling through a cooling towerhaving evaporative media, comprising: means for absorbing heat from afirst air stream into a second air stream, said absorbing means having afirst set of passageways and a second set of passageways; means fordirecting a first air stream having a flow rate between 10 and 80 poundsof dry air per square foot per minute (pda/ft²/min) and a relativehumidity at or above 90% through said first set of passageways of saidabsorbing means and for directing a second air stream having a flow ratebetween 10 and 80 pda/ft²/min and an entrance dry bulb temperature atleast five Fahrenheit degrees below the first stream through said secondset of passageways of said absorbing means each passageway of said firstset of passageways being separated from at least one passageway of saidsecond set of passageways by a thin heat conductive material; means forcondensing water out of said first air stream; means for capturing thewater condensed out of said first air stream; and means for preventingeffluent from passing around said means for absorbing heat using acovering disposed at the bottom of said means for absorbing heat. 13.The apparatus of claim 12 wherein the pressure inside each firstpassageway and each second passageway is between positive six millibarsand negative six millibars.
 14. The apparatus of claim 13, wherein saidfirst air stream is obtained from a cooling tower effluent.
 15. Theapparatus of claim 14, further comprising means for returning thecaptured water to the means for capturing water.
 16. The apparatus ofclaim 14, further comprising means for purifying the captured water. 17.The apparatus of claim 16, wherein the means for purifying the capturedwater performs a reverse osmosis process.
 18. The apparatus of claim 12wherein the first set of passageways are formed through cylindricaltubes and wherein the second set of passageways are the spaces aroundthe cylindrical tubes.
 19. The apparatus of claim 12, wherein the firstset of passageways and the second set of passageways are formed bysandwiching thin sheets together.
 20. The apparatus of claim 19, furthercomprising positively raised edges along two parallel edges of the thinsheet material and negatively raised edges along the two parallel edgesof the thin sheets perpendicular to the edges having the positivelyraised edges; said first passageways being formed by reversing twosheets and bonding the positively raised edges on one side together andthe positively raised edges on the other side together; and said secondpassageways being formed by reversing two sheets and bonding thenegatively raised edges on one side together and the negatively raisededges on the other side together.
 21. The apparatus of claim 20, whereinsaid first passageways can be oriented perpendicular to said secondpassageways by alternately bonding the negatively raised edges and thepositively raised edges in a set of thin sheets.
 22. The apparatus ofclaim 21, further comprising positively and negatively formed buttons inthe thin sheets for maintaining the passageways open under differentialpressure between said first passageways and said second passageways. 23.The apparatus of claim 22, wherein the positively formed buttons on afirst sheet press against the positively formed buttons on a firstadjacent sheet and the negatively formed buttons press against thenegatively formed buttons on a second adjacent sheet.
 24. The apparatusof claim 23, wherein the positively formed buttons are configured toreduce resistance to flow of the first air stream in a first directionand the negatively formed buttons are configured to reduce resistance toflow of the second air stream in a second direction.
 25. The apparatusof claim 24, wherein said thin sheets are made of a synthetic resinfilm.
 26. The apparatus of claim 25, wherein said thin sheets are madeof PVC.